1. Field of the Invention
The present invention relates to a photodetector, a photodetector array comprising photodetectors similar to the aforesaid photodetector, an object detector comprising the photodetector array, and an object detecting procedure. More particularly, the invention relates to a photodetector and a photodetector array suitable for detecting obstacles in proximity of a traveling vehicle or the like, an object detector comprising the photodetector array and an object detecting procedure.
2. Description of the Related Art
FIG. 1 shows a known photodetector described, for example, in C. C. Sun et al., "Quantum Electronics" Vol. 25, No. 5, pp.896-903 (1989). Shown in FIG. 1 are a light-absorptive n-type silicon substrate 211, a p.sup.+ -type silicon layer 212 formed in part of the surface of the n-type silicon substrate 211, a transparent polycrystalline silicon electrode 213, to which a bias voltage V.sub.c for setting the sensitivity of the photodetector, formed on the surface of the n-type silicon substrate 211 contiguously with the p.sup.+ -type silicon layer 212, a transparent silicon dioxide insulating film 214 insulating the polycrystalline silicon electrode 213 from the n-type silicon substrate 211, an output electrode 215, i.e. an opaque metal electrode, covering the p.sup.+ -type silicon layer 212, and a depletion layer 216. The depletion layer 216 is formed in the vicinity of the interface between the n-type silicon substrate 211 and the silicon dioxide insulating film 214, and also in the vicinity of the interface between the n-type silicon substrate 211 and the p.sup.+ -type silicon layer 212. The photodetector is a variable-sensitivity photodetector, in which the depth of the depletion layer 216 is changed by changing the bias voltage V.sub.c applied to the polycrystalline silicon electrode 213.
In operation, the bias voltage V.sub.c is applied to the polycrystalline silicon layer 213 to set the sensitivity of the photodetector. Since the photodetector has a MIS (metal-insulator semiconductor) construction formed by stacking the polycrystalline silicon electrode 213 and the silicon dioxide insulating film 214 on the silicon substrate 211, the depletion layer 216 is formed when the bias voltage V.sub.c applied to the polycrystalline silicon electrode 213 is a negative voltage. Accordingly, the depth of the depletion layer 216 varies with the variation of the negative bias voltage V.sub.c, and an appropriate negative voltage is applied to the polycrystalline silicon electrode 213 to set the photodetector for a photosensitivity. When light h.nu. is projected on the photodetector along a direction perpendicular to the surface of the photodetector, the light h.nu. travels through the silicon dioxide insulating film 214, the polycrystalline silicon electrode 213 and the silicon dioxide insulating film 214 in that order and falls on the depletion layer 216. When the wavelength of the incident light is shorter than the absorption edge of the semiconductor forming the n-type silicon substrate 211, a photocurrent I.sub.ph is produced, i.e., electron-hole pairs are created, in the depletion layer 216. The electrons gather in the pn junction of the p-type semiconductor, i.e., the p.sup.+ -type silicon layer 212, and the n-type semiconductor, i.e., the n-type silicon substrate 211, and the electrons appear on the output electrode 215.
The intensity of the photocurrent I.sub.ph produced by the photodetector is proportional to the thickness of the depletion layer 216; the intensity increases with the increase of the thickness of the depletion layer 216. The thickness of the depletion layer 216 is proportional to the magnitude of the bias voltage V.sub.c applied to the polycrystalline silicon electrode 211. Therefore, the intensity of the photocurrent I.sub.ph that flows through the photodetector, which corresponds to the sensitivity to incident light, can be changed by adjusting the bias voltage V.sub.c applied to the polycrystalline silicon electrode 211.
FIG. 2 shows a photodetector array, for example, a MOS image sensor, described in "Sensor Interfacing No. 2", Toranzista Gijutsu (Bessatsu), Apr. 1, 1983. Shown in FIG. 2 are photodiodes 206 serving as the pixels of the image sensor, MOS transistors 207 serving as vertical switches for reading pixel signals stored in the photodiodes 206, MOS transistors 208 serving as horizontal switches to read pixel signals along a horizontal direction from a bus connected to the vertical-switching MOS transistors 207, a vertical shift register 209 that supplies clock pulses for vertical reading to the vertical-switching MOS transistors 207, and a horizontal shift register 210 that applies clock pulses for horizontal reading to the horizontal-switching MOS transistors 208.
In operation, when light from an object falls on the photodiodes 206 arranged in a matrix, light-induced carriers are accumulated in each photodiode 206. Then, the vertical shift register 209 applies clock pulses sequentially to the horizontal lines of the vertical-switching MOS transistors 207 to turn on the MOS transistors 207 sequentially to make the photodiodes 206 deliver pixel signals onto the common bus. Then, the horizontal shift register 210 applies clock pulses to the horizontal-switching MOS transistors 208 to turn on the MOS transistors 208 sequentially to read the pixel signals on the vertical lines horizontally sequentially. Thus, two-dimensional image signals represented by the photocurrents produced by the two-dimensionally arranged photodiodes 206 are obtained.
Passive object detectors employing the foregoing image sensor are disclosed, for example, in Japanese Patent Publication Nos. 63-38085 and 63-46363, and Japanese Patent Laid-open No. 63-52300. Each of these known object detectors is provided, for example, with two sets each of the image sensor and an input optical system to sense the scenery spreading before the object detector to acquire image data. The object detector processes the image data to detect an object and obtains three-dimensional data including the distance to the object.
FIG. 3 is a block diagram of a known object detector, such as an optical radar, described in, for example, "Laser Handbook", Asakura Shoten, pp.644-676 (1973) or Japanese Patent Publication No. 60-4011. Referring to FIG. 3, there is shown a pulse generator 201 that generates clock pulses, i.e., a reference pulse signal, a light-emitting unit 202 comprising a light-emitting device, such as a semiconductor laser, that generates light pulses, in response to the clock pulses generated by the pulse generator 201, a driving circuit for driving the light-emitting device, and a lens system. The object detector also includes a photodetector unit 203 for receiving light pulses reflected from the object, comprising the lens system through which light signals, i.e., the reflected light pulses, are received, light sensing devices, such as photodiodes, for converting received light signals into electrical signals, a bias voltage generator that generates a bias voltage for biasing the light sensing devices, an amplifier 204, such as a wide-band amplifier, for amplifying the output electric signals of the photodetector unit 203, and a rangefinder 205 that counts propagation delay times on the basis of the clock pulses generated by the pulse generator 201 to calculate the distance to the object.
In operation, the pulse generator 201 is driven and generates clock pulses at intervals far longer than a time interval corresponding to a maximum measurable distance. The clock pulses are applied as driving pulses to the light-emitting unit 202, and then the light emitting unit emits light pulses L.sub.t using the light-emitting device, such as a semiconductor laser, according to the driving pulses. The light pulses L.sub.t are projected through a lens or the like on a reflecting object. The photodetector unit 203 receives reflected light pulses L.sub.r reflected from the reflecting object, and converts the reflected light pulses L.sub.r into electric signals by photoelectric conversion by light-sensitive devices, such as photodiodes. The amplifier 204 amplifies the output electric signals of the photodetector unit 203 to electric signals on a predetermined level and gives the amplified electric signals to the rangefinder 205. The rangefinder 205 determines the distance to the reflecting object by measuring the time interval between transmission of the pulse and reception of the reflected pulse. The optical radar is an active object detector that transmits a microwave or an infrared laser light into a three-dimensional space, receives the reflected signal reflected from an object in the three-dimensional space by the lightsensing unit 204, determines the distance to the object, and detects the object and the relative speed between the object and the optical radar.
In the conventional variable-sensitivity photodetector thus constructed, the photocurrent I.sub.ph flows always in the fixed direction, and the sensitivity can be varied only in the positive direction corresponding to the direction of flow of the photocurrent I.sub.ph ; that is, since the sensitivity of the variable-sensitivity photodetector cannot be varied in the negative direction or negatively weighted, the object detector employing this variable-sensitivity photodetector is incapable of carrying out effective processing, such as calculation of the difference between the pixels of an image. Although the conventional variable-sensitivity photodetector is illuminated entirely by the incident light, the variable-sensitivity photodetector is able to use the energy of only part of the incident light falling on the depletion layer and is unable to use the energy of all the incident light effectively. Since the variable-sensitivity photodetector receives all the incident radiations having wavelengths shorter than the absorption edge of the semiconductor, the variable-sensitivity photodetector is incapable of effectively using the respective characteristics of the component radiations having different wavelengths of the incident light, and, since the variable-sensitivity photodetector has no threshold action with the intensity of the incident light, the variable-sensitivity photodetector is unable to discriminate between the useful signal components and noise components of the incident light.
Still further, since the conventional photodetector array has a complicated construction, it is difficult to increase the number of pixels to process two-dimensional image data, and the photodetector array needs a light source, such as a light-emitting device, to process three-dimensional image data including distance data. The object detector for processing three-dimensional image data including distance data, comprises, in combination, the conventional photodetector array and a light-emitting device needing a complicated optical system which is difficult to align.
The conventional object detector employing an active optical radar is incapable of obtaining information about an object other than the distance to the object, such as the width and the height of the object. When applied, for example, to an automobile, this conventional object detector could not achieve highly reliable obstacle detection. Since a passive object detector employing an image sensor collects a large number of pieces of image information successively, the passive object detector cannot be applied to function where real-time image detection is required.